To spread infection, many enveloped viruses are released from infected cells by budding off of the plasma membrane. Recently, an interferon-induced membrane protein, tetherin/ BST2, has been identified as a potent host antiviral factor inhibiting the release of a range of enveloped viruses, such as HIV-1. Tetherin directly tethers viruses to the host plasma membrane and interacts with the cellular endocytic machinery for viral internalization and degradation. HIV-1 viral protein U (Vpu) antagonizes tetherin by recruiting a host cellular E3 ubiquitin ligase to polyubiquitinate tetherin, thereby directing it to host pathways recognizing ubiquitin as a trafficking signal such as proteosomal/lysosomal degradation and/or endosomal segregation. The overall goal of our study is to establish the mechanisms by which tetherin restricts the release of enveloped viruses and the mechanisms by which viruses antagonize tetherin. We will achieve our goal by using a multidisciplinary approach combining cutting-edge biochemical and biophysical techniques, functional virology, cryo-electron microscopy and electron tomography, and X-ray crystallographic methods to determine the biochemical and structural principles of tetherin function, to investigate viral-cellular interactions in HIV-1 antagonization of tetherin, and to test their functional significance. Our work will allow for the elucidation of a major host immune defense mechanism and significantly advance our understanding of a diverse range of host-viral interplays. Information derived from our studies will generate a framework for the development of new therapeutic interventions of HIV and other viruses. Moreover, the experimental systems devised for our research project will provide valuable new tools for the studies of host-pathogen relationships.